Hitherto, stamping has been the only technique for displaying scale marks or other patterns on fluororesin-coated articles, in which technique a fluororesin-coated article is physically impressed by pressing or other means to produce an impressed mark, etc., or a base physically impressed beforehand is coated with a fluororesin coating composition to utilize the shadows resulting from the impressing. However, this technique has serious problems as follows.
First, in the case of the former method in which a fluororesin-coated article is physically impressed by pressing or other means, since the impressed part of the coated article is deformed at an exceedingly high curvature, the fluororesin coating layer is stretched simultaneously and is apt to peel off the base or develop pinholes, cracks, or other defects. Through such pinholes or cracks, water or rice broth penetrates into the fluororesin coating film to cause corrosion of the base, peeling of the coating layer, discoloration, etc.
The second problem is that the display is not easily recognizable because the recognition thereof is based only on shadows resulting from impressing. This problem is becoming very severe when the product of the fluororesin-coated article has multiple functions. For example, since inner pots of rice cookers, in particular, have come to be required to have multiple functions and, with this trend, displays such as scale marks are not limited to one kind, i.e., those for ordinary rice cooking, but range over various kinds including those for cooking rice together with red beans, for cooking rice gruels seasoned with miso or soy, and for cooking glutinous rice. Therefore, there are increasing numbers of cases in which three or four scales are impressed on one product of the fluororesin-coated article such as inner pot of rice cooker.
As expedients for overcoming these problems, various scale-displaying techniques have been proposed. For example, JP-A-60-232116 discloses a technique of producing a scale display by coating impressed parts of a pot with a fluororesin having a color different from that of the pot. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) However, this technique has a drawback that it requires troublesome steps and is, hence, industrially disadvantageous. In addition, the above technique has a further disadvantage that since the depressed parts of the impressed parts remain almost as it is in the pot produced, the presence of the scale marks is inconvenient for cleaning the pot, as in the case of the pots produced by prior-art techniques.
Further improvements of the above-described technique are disclosed in JP-A-1-170428 and JP-A-2-36815. In these improved displaying techniques, scale marks or other patterns are formed on a pot having a fluororesin coating but bearing no impressed marks, by printing the pot on its inner side with a fluororesin having a color different from that of the coating. This method, however, also is defective in the following two points.
First, the scale marks printed may disappear by wear because the display constitutes the outermost layer.
Second, the existing acute problems accompanying the use of a conventional fluororesin coating composition to form such a scale display have not been fully overcome, such as problems concerning the cissing or running of the coating composition on the fluororesin coating layer, adhesion to the base or fluororesin coating layer, and the flow characteristics of the fluororesin coating composition as a printing-use coating fluid, i.e., as a printing ink.
Specifically, this method is, for example, defective in that (1) when a commercially available fluororesin coating composition is used to form printed marks on a fluororesin layer, the coating composition applied suffers cissing due to the water repellency of the fluororesin layer, so that clear printed marks are difficult to form, and (2) if printing is conducted on a base made of Al or the like, it is difficult to obtain a practically sufficient adhesion strength.
The above problem (1) of coating composition cissing may be solved by using a fluororesin coating composition which has been thickened by increasing the solid content thereof to a value considerably higher than those of commercially available fluororesin coating compositions. However, use of such a coating composition poses new problems, for example, that the high viscosity of the coating composition results in increased printed mark thicknesses to give projected printed marks, and that the coating composition necessarily has poor leveling properties, which are characteristic of dispersion-type coating compositions having high solid contents, and the resulting printed surface has poor smoothness and the printed marks have uneven thicknesses. Although the problem of the increase in printed mark thickness may be overcome by thickening a fluororesin coating composition by incorporating a thickening agent or other additive, the thus-thickened composition comes to have structural viscosity such as jelly-like viscosity and, as a result, uniform printing is also difficult. Problem (2) of insufficient adhesion above may be solved by incorporating into a fluororesin a primer or adhesive ordinarily used for this purpose. However, printed marks formed from such a coating composition necessarily have a brownish color attributable to the adhesion-improving ingredient, so that the color of the printed marks are limited to dark ones such as black.
As apparent from the above, a smooth and flat display, such as scale marks, which is desired in the field of fluororesin-coated articles has been unable to be attained by a mere combination of common techniques or by an improvement that can be easily made by a skilled artisan in the art.
As inner pots for rice cookers, inner vessels for thermoses, frying pans, etc. there have been widely used those which have been coated with a fluororesin on the inner surface thereof. Heretofore, these products have been prepared by coating the entire surface of a substrate such as aluminum plate with a fluororesin coating composition free of or containing a pigment by spin coating process (see "Introduction to Coating Apparatus and Process", 2nd edition, K.K. Sogo Gijutu Center, Apr. 2, 1990, page 197), drying and sintering the coated substrate, and, if the fluororesin coating contains a pigment, coating the material further with a pigment-free fluororesin coating, drying and sintering the coated substrate, and then mechanically shaped by pressing.
However, the spin coating process has manufacturing disadvantages that it causes a great loss of the coating composition, requiring a recovery system. This coating process is also principally disadvantageous, since the resulting coating is thicker at the center and the edge than the other area. Further, the area to be coated cannot be specified, causing undesirable areas to be coated. Moreover, it is difficult to obtain a coating having a thickness of not less than 20 .mu.m by a single coating.
On the other hand, the screen printing process (as described, e.g., in Introduction to Coating Apparatus and Process, 2nd edition, K.K. Sogo Gijutu Center, Apr. 2, 1990, page 197) is used in all printing fields today. This printing process causes little loss of the coating composition and can provide a coating in any pattern. This printing process requires only a light weight and inexpensive apparatus. Thus, the screen printing process provides various practical conveniences.
However, if the screen printing process is applied to the coating with a fluororesin, a coating composition containing a fluororesin must be optimized for the screen printing. Since a fluororesin is generally insoluble in a solvent, a coating composition for screen printing containing a fluororesin cannot be obtained in accordance with the conventional manner.
Application of a fluororesin coating composition to the screen printing process is described in JP-B-2-61308 (The term "JP-B" as used herein means an "examined Japanese patent publication ")), which discloses a process for the formation of a decoration on a polytetrafluoroethylene (PTFE) coating. In some detail, this process comprises coating a metallic substrate with an aqueous dispersion of PTFE, drying the coated substrate, screen-printing a composition containing a fluorocarbon powder before sintering, and then baking for sintering the two layers at the same time. However, there is no reference to a technique for directly coating a metallic substrate with a fluororesin by screen printing.
JP-A-3-217470 describes a process which comprises printing on a metallic substrate an ink composition obtained by mixing a resin that undergoes thermal decomposition at 450.degree. C. or lower with a fluororesin powder, etc., and then decomposing the resin under heating. However, this process requires complete decomposition of a significant quantity of the resin under heating, inevitably requiring a prolonged processing at a high temperature. Therefore, no matter how heat-resistant it is, the fluororesin inevitably undergoes thermal decomposition during this process even though it is heat-resistant, adversely affecting physical properties such as abrasion resistance and nontackiness.
Further, all the foregoing processes are aimed at obtaining a decorative effect by screen-printing a pattern such as character and graphic but are not disclosed as a process for coating the entire surface of a substrate with a fluororesin.
Therefore, no screen printing processes and coatings have been heretofore disclosed for the formation of a transparent uniform fluororesin coat having a thickness of not less than 20 .mu.m on substantially the entire surface of a substrate.